1. Field of the Invention
The present invention relates to a process for manufacturing wet-felted and thermally bonded porous structures which can be used as filters. The invention also relates to the fibrous felted porous structure formed by the process.
2. Background of the Related Art
Felted porous structures such as filters are typically manufactured by xe2x80x9caccretion.xe2x80x9d In an accretion process, a homogeneous bath or slurry of the material to be accreted is mixed and a vacuum forming process is used to create a formed media from the materials in the slurry. In one vacuum forming method often used, a perforated mandrel is immersed in the bath or slurry, and a vacuum is pulled on the mandrel causing a layer of material to accrete on the outer surface of the mandrel to create a formed media. The mandrel is withdrawn from the bath or slurry and the formed media is dried. Depending on the intended use, the formed media may be densified or may be cut to a desired shape.
In some prior applications of the accretion method, a slurry is mixed in water with a combination of a fibers and a thermoplastic or thermosetting binding material. Other materials may be added to the slurry to impart desired filtering characteristics into the formed media. Accretion of the material in the slurry may be performed using a vacuum mandrel to create a formed porous media on the mandrel. In some applications, a filter core is placed over the vacuum mandrel and the accretion process creates a formed media covering the filter core.
After the accretion step is complete, the formed media is dried and bonded by heating the formed media to a temperature above the melting or curing temperature of the binding material. This dries the formed media and bonds the primary media and wet strength agent by melting or curing the binding material. As the formed media is cooled, the binding material solidifies and binds the formed media.
While several variations of this process have been described previously, in each case the drying and bonding of the formed media is performed by heating the accreted formed media to a temperature above the melting point of the binding agent in a single step. For example, in U.S. Pat. No. 4,032,457 to Matchett, a multiple phase filter is described which is made by combining fibers with resin binders and active particles to form slurries of varying compositions. The filter described by Matchett is formed by dipping a perforated mandrel is a slurry, drawing a vacuum on the mandrel to accrete material from the slurry on the mandrel, and repeating these steps in successive slurries of varying compositions to form a media with multiple phases. After formation, the multiple phase media is dried and bonded in a single step by heating the formed media to a temperature above the melting temperature of the binder.
In U.S. Pat. No. 4,620,932 to Howery, et al., a one piece filter constructed by saturation of a base matrix material with a hydrophilic terpolymeric material is described. Saturation of the base material is accomplished by spraying, depth coating, or dipping the base matrix in the hydrophilic terpolymer material. The saturated material is then initially dried at a temperature of 160-250 degrees Fahrenheit to remove 40-60 percent of the moisture from the material, with second stage drying performed at a lower temperature of 100-160 degrees Fahrenheit, and final drying occurring in a third stage at 65-90 degrees Fahrenheit. Thus, Howery describes a process in which the material is initially heated to a high temperature, with drying being completed in steps with successively lower temperatures.
Combinations of base fiber materials and binding materials have also been used to produce non-woven fabrics and other materials. For example, U.S. Pat. No. 5,393,601 to Heinrich, et al., describes a non-woven material formed by combining aramid fibers with a melt binder made of thermoplastic aramids The melt binder has a melting point below the melting point of the aramid fibers. The aramid fibers and the melt binder are mixed in water, the aqueous suspension is placed on a sieve tray, the water is separated off, and the remaining fibers are heated to a temperature above the melting temperature of the melt binder to dry and bind the fibers.
In all of these prior methods, the formed media are initially heated to a temperature above the melting temperature of the binder in a single step. Combining drying and bonding in a single step process can sometimes lead to uneven bonding of the fibers in the formed media. Uneven bonding is particularly disadvantageous where the formed media is used as a filtering media, because uneven bonding can reduce filter performance and quality.
Accordingly, the present invention overcomes the drawbacks and disadvantages of the prior art through a novel process for producing wet felted and thermally bonded porous media that results in improved filter quality and performance.
The present invention provides, in one aspect, an improved process for producing wet-felted and thermally bonded porous media, and, in a second aspect, provides an improved wet-felted and thermally bonded porous media made by the process. The invention comprises a process in which a primary media, a wet strength agent, and a binding agent are combined in a liquid, typically water, to form a slurry. Other materials may be added to the slurry to impart desired characteristics to the porous media. A vacuum forming process is used to accrete the materials in the slurry into a formed media. The formed media is then dried and bonded in a two-step process.
The formed media is first dried a temperature below the melting or curing temperature of the binding agent. The drying may be performed under vacuum to facilitate rapid drying. The drying step is continued for a sufficient time to remove substantially all of the water from the formed media.
After the formed media has been dried, it is bonded by heating the formed media to a temperature above the melting or curing temperature of the binding agent. This second stage of heating may be performed under a vacuum to draw heated gas through the porous media. If a vacuum is used, the vacuum pressure during the bonding step may be different from the vacuum pressure during the drying step. During the second stage of heating, the binding agent melts or cures to bind the formed media. When the bonding step is completed, the formed media is cooled, and the binding agent resolidifies. After cooling, the formed media may be cut into any desired shape or size.
One advantage of the present invention is better control over the bonding step. This results in more even bonding of the formed media, which improves the quality and filtration performance of the formed media over that of media manufactured by the previously known processes.
It will be readily appreciated by those skilled in the art that the characteristics of the formed media can be varied as desired by using different primary media or wet strength agents, or by supplying additional components in the slurry to provide desired characteristics. Other advantages of the present invention will become more readily apparent from the following description of the drawings taken in conjunction with the detailed description of the preferred embodiments.